Liquid-fluid extraction is a unitary operation commonly used in industrial chemistry and in analytical chemistry, and it consists in causing mass of at least one solute to be transferred between a liquid first phase and a fluid (liquid or gaseous) second phase that is immiscible with the first. It is well known that in order to encourage such transfer, it is necessary to maximize the ratio of the contact area between the phases divided by the volume of the phases. Typically, that is achieved by dispersing one of the phases in the other, e.g. in a beaker and using a magnetic stirrer, followed by the phases separating out by settling.
International patent document WO 96/12540 discloses a method of transferring solute between two non-miscible fluid phases through a plane porous membrane. The transfer of solute from one phase to the other takes place through the pores in the membrane, without which the plane interface between the two fluids would be unstable. The two phases are recovered independently since they are not mixed together at any time.
The article “Analytical chemistry in a drop. Solvent extraction in a microdrop” by H. Liu and P. Dasgupta, published in Analytical Chemistry, Volume 68, page 1817 on Jun. 1, 1996 discloses an analytical application of liquid-liquid extraction at microliter scale. In that article, a droplet of chloroform having a volume of about 1 microliter (μL) is suspended inside a larger drop of a continuously renewed aqueous solution. A solute passes from the aqueous phase to the organic phase and is detected by laser spectrometry. That method requires only very small quantities of solvents and substances for analysis, and it makes it possible to obtain a very high surface/volume ratio. However it is complex to implement since it requires tricky assembly of discrete microfluidic elements.